Wet Compression Test
A ‘Wet’ Compression test will help you to further nail down the reason for the low cylinder pressure or pressures you recorded in the ‘dry’ compression test.
To be a bit more specific, a ‘Wet’ compression test will help you find out if the low compression readings are caused by worn piston rings or worn cylinder head valves.
To do a ‘Wet’ compression test, you'll need to add a small amount of engine oil (about 1-2 teaspoons) to the cylinder that had low compression and recheck its compression with the compression tester.
Depending on whether the compression pressure rises (on your compression tester) or not, you'll be able to say that the problem lies in the piston's rings or in the cylinder head valves.
OK, this is what you need to do:
Add a small amount of engine oil to the cylinder that reported low compression or no compression in the ‘dry’ compression test.
- The amount should be about 1 to 2 tablespoons of oil.
Install the compression tester onto the cylinder.
- Do not use any type of tool to tightened the compression tester. Hand tight is fine.
When all is set up, have your helper crank the engine.
You'll get one of two results, either the compression value will go up (from the one you recorded before) or it will stay the same.
Let's examine your compression test results:
CASE 1: The compression value shot up. This tells you that the Piston Compression Rings are worn out and thus the problem is in the bottom end.
CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.
If this info really saved the day, buy me a beer!